A liquid crystal display device is used in various electronic appliances such as a television receiver and a personal computer. Generally, it is preferred that a liquid crystal display device is low in power consumption. In particular, a liquid crystal display device used in portable electronic appliances (e.g., a mobile telephone, a handheld computer) is strongly requested to achieve low power consumption.
As one of methods for reducing power consumption in a liquid crystal display device, there has been known a method for switching a liquid crystal driving frequency. For example, in a case where a liquid crystal display device is used in a handheld computer, when a state in which the handheld computer receives no manipulation input from a user continues for a predetermined time or more, a liquid crystal driving frequency may be set to be lower than that in a normal state. When the liquid crystal driving frequency is set to be low, power consumption is reduced considerably although a cycle of updating a screen becomes long.
On the other hand, a liquid crystal has a characteristic in that degradation takes place in a short time by application of a direct-current voltage. For this reason, the liquid crystal display device performs alternating-current driving for switching a polarity of a liquid crystal application voltage at every predetermined cycle. Moreover, when an effective value of the liquid crystal application voltage upon application of a positive polarity voltage (hereinafter, referred to as “in a positive polarity”) is different from an effective value of the liquid crystal application voltage upon application of a negative polarity voltage (hereinafter, referred to as “in a negative polarity”), flicker occurs at the screen. In order to prevent this flicker, a process of adjusting a voltage to be applied to a common electrode (hereinafter, referred to as a common voltage Vcom) is performed to set the effective value of the liquid crystal application voltage in the positive polarity to be equal to the effective value of the liquid crystal application voltage in the negative polarity.
With reference to FIGS. 10 and 11, description will be given of the adjustment of the common voltage Vcom. FIG. 10 is an equivalent circuit diagram of a pixel circuit included in a liquid crystal display device. In the pixel circuit 11 shown in FIG. 10, a TFT (Thin Film Transistor) 12 has a gate terminal connected to a gate line Gj, a source terminal connected to a source line Si, and a drain terminal connected to a first electrode of a liquid crystal capacitor 13 and a first electrode of an auxiliary capacitor 14. The common voltage Vcom is applied to a second electrode of the liquid crystal capacitor 13, and an auxiliary voltage Vcs is applied to a second electrode of the auxiliary capacitor 14.
FIG. 11 is a signal waveform chart showing change of terminal voltages of the TFT 12. In order to write a voltage in accordance with display data to the pixel circuit 11, a high-level voltage Vgh is applied to the gate line Gj, and a positive polarity voltage or a negative polarity voltage, both in accordance with the display data, is applied to the source line Si. When a gate voltage Vg turns into Vgh, the TFT 12 turns into an ON state, so that a drain voltage Vd becomes equal to a source voltage Vs.
Thereafter, when a low-level voltage Vgl is applied to the gate line Gj, the TFT 12 turns into an OFF state. Since a parasitic capacitor is provided between the gate and the drain of the TFT 12, when the gate voltage Vg changes from Vgh to Vgl, the drain voltage Vd drops by a predetermined amount. A drop amount ΔV in such a case is referred to as a pull-in voltage or a feed-through voltage, and is expressed by the following equation (1).ΔV=Vgp-p×Cgd/(Clc+Ccs+Cgd)  (1)
In the equation (1), Vgp-p represents a gate voltage amplitude (=Vgh−Vgl), Clc represents a capacitance value of the liquid crystal capacitor 13, Ccs represents a capacitance value of the auxiliary capacitor 14, and Cgd represents a capacitance value of the parasitic capacitor between the gate and the drain of the TFT 12.
After the TFT 12 turns into the OFF state, a leak current flows through the TFT 12; therefore, the drain voltage Vd gradually rises or drops to approach the common voltage Vcom. This state continues until the high-level voltage Vgh is applied to the gate line Gj after one frame period.
In the pixel circuit 11, the liquid crystal capacitor 13 corresponds to a liquid crystal element. A transmittance of a liquid crystal panel is determined based on the effective value of the liquid crystal application voltage, that is, an effective value of a difference between the drain voltage Vd and the common voltage Vcom (a diagonally shaded portion in FIG. 11). Accordingly, the common voltage Vcom is adjusted such that an effective voltage Vrms(p) in the positive polarity becomes equal to an effective voltage Vrms(n) in the negative polarity. Thus, the transmittance of the liquid crystal panel in the positive polarity is set to be equal to the transmittance of the liquid crystal panel in the negative polarity, and a difference in luminance is eliminated. As a result, flicker can be prevented.
In relation to the present invention, Patent Document 1 describes a technique of changing a common voltage or a signal voltage in accordance with a length of a write holding time. Moreover, Patent Document 2 describes a technique of changing both a gate-on voltage and a common voltage in accordance with a horizontal synchronous frequency.
[Patent Document 1] Japanese Laid-Open Patent Publication No. 2002-116739
[Patent Document 2] Japanese Laid-Open Patent Publication No. 2001-13930